Aqueous emulsion polymerization of fluorinated monomers using a perfluoropolyether surfactant

ABSTRACT

The invention relates to an aqueous emulsion polymerization of fluorinated monomers using perfluoropolyethers of the following formula (I) or (II). In particular, the perfluoropolyether surfactants correspond to formula (I) or (II) 
 
CF 3 —(OCF 2 ) m —O—CF 2 —X  (I) 
wherein m has a value of 1 to 6 and X represents a carboxylic acid group or salt thereof, 
 
CF 3 —O—(CF 2 ) 3 —(OCF(CF 3 )—CF 2 ) z —O-L-Y  (II) 
 
wherein z has a value of 0, 1, 2 or 3, L represents a divalent linking group selected from —CF(CF 3 )—, —CF 2 — and —CF 2 CF 2 — and Y represents a carboxylic acid group or salt thereof. The invention further relates to an aqueous dispersion of a fluoropolymer having the aforementioned perfluoropolyether surfactant(s).

CROSS-REFERENCE TO RELATED APPLICATION

This application claims priority to Great Britain Patent Application No.GBO514387.0, filed on Jul. 15, 2005, herein incorporated by reference inits entirety.

The present invention relates to the aqueous emulsion polymerization offluorinated monomers to produce fluoropolymers.

BACKGROUND OF THE INVENTION

Fluoropolymers, i.e. polymers having a fluorinated backbone, have beenlong known and have been used in a variety of applications because ofseveral desirable properties such as heat resistance, chemicalresistance, weatherability, UV-stability etc. The various fluoropolymersare for example described in “Modern Fluoropolymers”, edited by JohnScheirs, Wiley Science 1997. Commonly known or commercially employedfluoropolymers include polytetrafluoroethylene (PTFE), copolymers oftetrafluoroethylene (TFE) and hexafluoropropylene (HFP) (FEP polymers),perfluoroalkoxy copolymers (PFA), ethylene-tetrafluoroethylene (ETFE)copolymers, terpolymers of tetrafluoroethylene, hexafluoropropylene andvinylidene fluoride (THV) and polyvinylidene fluoride polymers (PVDF).Commercially employed fluoropolymers also include fluoroelastomers andthermoplastic fluoropolymers.

Several methods are known to produce fluoropolymers. Such methodsinclude suspension polymerization as disclosed in e.g. U.S. Pat. No.3,855,191, U.S. Pat. No. 4,439,385 and EP 649863; aqueous: emulsionpolymerization as disclosed in e.g. U.S. Pat. No. 3,635,926 and U.S.Pat. No. 4,262,101; solution polymerization as disclosed in U.S. Pat.No. 3,642,742, U.S. Pat. No. 4,588,796 and U.S. Pat. No. 5,663,255;polymerization using supercritical CO₂ as disclosed in JP 46011031 andEP 964009 and polymerization in the gas phase as disclosed in U.S. Pat.No. 4,861,845.

Currently, the most commonly employed polymerization methods includesuspension polymerization and especially aqueous emulsionpolymerization. The aqueous emulsion polymerization normally involvesthe polymerization in the presence of a fluorinated surfactant, which isgenerally used for the stabilization of the polymer particles formed.The suspension polymerization generally does not involve the use ofsurfactant but results in substantially larger polymer particles than incase of the aqueous emulsion polymerization. Thus, the polymer particlesin case of suspension polymerization will quickly settle out whereas incase of dispersions obtained in emulsion polymerization generally goodstability over a long period of time is obtained.

An aqueous emulsion polymerization wherein no surfactant is used hasbeen described in U.S. Pat. No. 5,453,477, WO 96/24622 and WO 97/17381to generally produce homo- and copolymers of chlorotrifluoroethylene(CTFE). For example, WO 97/17381 discloses an aqueous emulsionpolymerization in the absence of a surfactant wherein a radicalinitiator system of a reducing agent and oxidizing agent is used toinitiate the polymerization and whereby the initiator system is added inone or more further charges during the polymerization. So-calledemulsifier free polymerization has further been disclosed in WO 02/88206and WO 02/88203. In the latter PCT application, the use of dimethylether or methyl tertiary butyl ether is taught to minimize formation oflow molecular weight fractions that may be extractable from thefluoropolymer. WO 02/88207 teaches an emulsifier free polymerizationusing certain chain transfer agents to minimize formation of watersoluble fluorinated compounds. An emulsifier free polymerization isfurther disclosed in RU 2158274 for making an elastomeric copolymer ofhexafluoropropylene and vinylidene fluoride.

Notwithstanding the fact that emulsifier free polymerizations are known,the aqueous emulsion polymerization process in the presence offluorinated surfactants is still a desirable process to producefluoropolymers because it can yield stable fluoropolymer particledispersions in high yield and in a more environmental friendly way thanfor example polymerizations conducted in an organic solvent. Frequently,the emulsion polymerization process is carried out using aperfluoroalkanoic acid or salt thereof as a surfactant. Thesesurfactants are typically used as they provide a wide variety ofdesirable properties such as high speed of polymerization, goodcopolymerization properties of fluorinated olefins with comonomers,small particle sizes of the resulting dispersion can be achieved, goodpolymerization yields i.e. a high amount of solids can be produced, gooddispersion stability, etc. However, environmental concerns have beenraised against these surfactants and moreover these surfactants aregenerally expensive.

Alternative surfactants to the perfluoroalkanoic acids or salts thereofhave also been proposed in the art for conducting the emulsionpolymerization of fluorinated monomers.

For example, surfactants of the general formula R_(f)—C₂H₄—SO₃M, whereinR_(f) represents a perfluorinated aliphatic group and wherein Mrepresents a cation, have been disclosed in U.S. Pat. No. 5,789,508,U.S. Pat. No. 4,025,709, U.S. Pat. No. 5,688,884 and U.S. Pat. No.4,380,618.

U.S. Pat. No. 5,763,552 discloses partially fluorinated surfactants ofthe general formula R_(f)—(CH₂)_(m)—R′_(f)—COOM wherein R_(f) representsa perfluoroalkyl group or a perfluoroalkoxy group of 3 to 8 carbonatoms, R′_(f) represents a perfluoroalkylene of 1 to 4 carbon atoms andm is 1-3.

U.S. Pat. No. 4,621,116 discloses perfluoroalkoxy benzene sulphonicacids and salts thereof in the aqueous emulsion polymerization offluorinated monomers.

U.S. Pat. No. 3,271,341 teaches perfluoropolyethers of the generalformula:F—(CF₂)_(m)—O—[CFX—CF₂—O]_(n)—CFX—COOAwherein m is 1 to 5, X is F or CF₃, A is a monovalent cation and n is 0to 10. The perfluoropolyethers are taught as emulsifiers in the emulsionpolymerization of ethylenically unsaturated monomers.

U.S. Publication No. 2005/0090613 discloses fluorinated polyethers ofthe formula:F—(CF₂)_(m)—O—[CFX—CF₂—O]_(n)—CFX—COOAwherein m is 3 to 10, X is F or a perfluoroalkyl group, n is 0, 1 or 2and A is the counter ion of the carboxylic anion. These polyethers aretaught as emulsifiers in the emulsion polymerization of fluorinatedolefins.

The use of perfluoropolyethers having neutral end groups in an aqueousemulsion polymerization is disclosed in U.S. Pat. No. 4,864,006, U.S.Pat. No. 4,789,717 and EP 625526. For example U.S. Pat. No. 4,864,006and EP 625526 disclose the use of microemulsion prepared fromperfluoropolyethers having neutral end groups in an aqueous emulsionpolymerization of fluorinated monomers. In a particular embodiment, acertain perfluoropolyether having carboxylic end groups is taught toemulsify the neutral perfluoropolyether.

EP 1,334,996 discloses certain perfluoropolyethers having carboxylicacid groups or salts thereof at both end groups, i.e. theperfluoropolyethers are bifunctional. The perfluoropolyethers are taughtfor use in aqueous dispersions of fluoropolymers and in the preparationof such dispersion by aqueous emulsion polymerization.

WO 00/71590 teaches the use of a combination of perfluoropolyethersurfactants having a carboxylic acid group or salt thereof with afluoroalkyl carboxylic acid or sulphonic acid or salt thereof. It istaught that the perfluoropolyether surfactants on their own are not verypowerful surfactants.

SUMMARY OF THE INVENTION

It would now be desirable to find an alternative emulsion polymerizationprocess in which the use of perfluoroalkanoic acids and salts thereof asa fluorinated surfactant can be avoided. In particular, it would bedesirable to find an alternative surfactant or dispersant, in particularone that is more environmentally friendly, for example has a lowtoxicity and/or shows no or only little bioaccumulation. It would alsobe desirable that the alternative surfactant has good chemical andthermal stability enabling polymerization over a wide range ofconditions of for example temperature and/or pressure. Desirably, thealternative surfactant or dispersant allows for a high polymerizationrate, good dispersion stability, good yields, good copolymerizationproperties and/or the possibility of obtaining a wide variety ofparticle sizes including small particle sizes. The properties of theresulting fluoropolymer should generally not be negatively influencedand preferably would be improved. Desirably, the resulting dispersionshave good or excellent properties in coating applications and/orimpregnation of substrates, including for example good film formingproperties. It would further be desirable that the polymerization can becarried out in a convenient and cost effective way, preferably usingequipment commonly used in the aqueous emulsion polymerization offluorinated monomers. Additionally, it may be desirable to recover thealternative surfactant or dispersant from waste water streams and/or toremove or recover the surfactant from the dispersion subsequent to thepolymerization. Desirably, such recovery can proceed in an easy,convenient and cost effective way.

It has been found that perfluoropolyethers of the following formula (I)or (II) are effective in the aqueous emulsion polymerization, even whenused without the addition of other surfactants such as perfluoroalkanoicacids and salts thereof. In particular, the perfluoropolyethersurfactants correspond to formula (I) or (II)CF₃—(OCF₂)_(m)—O—CF₂—X  (I)wherein m has a value of 1 to 6 and X represents a carboxylic acid groupor salt thereof;CF₃—O—(CF₂)₃—(OCF(CF₃)—CF₂)_(z)—O-L-Y  (II)wherein z has a value of 0, 1, 2 or 3, L represents a divalent linkinggroup selected from —CF(CF₃)—, —CF₂— and —CF₂CF₂— and Y represents acarboxylic acid group or salt thereof. Examples of carboxylic acid saltsinclude sodium, potassium and ammonium (NH₄) salts.

Thus, in one aspect, the invention relates to a method for making afluoropolymer comprising an aqueous emulsion polymerization of one ormore fluorinated monomers wherein said aqueous emulsion polymerizationis carried out in the presence of a perfluoropolyether as an emulsifier,said perfluoropolyether being selected from the group consisting ofperfluoropolyethers according to above formula (I), perfluoropolyethersaccording to above formula (II) and mixtures of perfluoropolyethersaccording to formula (I) and/or (II).

In a further aspect, the invention relates to an aqueous dispersion of afluoropolymer comprising a perfluoropolyether as an emulsifier, saidperfluoropolyether being selected from the group consisting ofperfluoropolyethers according to above formula (I), perfluoropolyethersaccording to above formula (II) and mixtures of perfluoropolyethersaccording to formula (I) and/or (II).

Since the aqueous emulsion polymerization can be carried out without theneed for using a perfluoroalkanoic acid, dispersions can be readilyobtained that are free of such perfluoroalkanoic acids or salts thereof.Thus, in a further aspect, the present invention relates to an aqueousdispersion of a fluoropolymer comprising a perfluoropolyether selectedfrom the group consisting of perfluoropolyethers according to aboveformula (I), perfluoropolyethers according to above formula (II) andmixtures of perfluoropolyethers according to formula (I) and/or (II) asan emulsifier and wherein the aqueous dispersion is free ofperfluorinated alkanoic acids or salts thereof.

DETAILED DESCRIPTION OF THE INVENTION

The resulting dispersions can be used in a variety of applicationsincluding coating and impregnation of substrates. Generally, a non-ionicsurfactant should be added to the dispersion for such applications.Accordingly, the invention in a further aspect relates to aqueousdispersions of a fluoropolymer comprising a perfluoropolyether selectedfrom the group consisting of perfluoropolyethers according to aboveformula (I), perfluoropolyethers according to above formula (II) andmixtures of perfluoropolyethers according to formula (I) and/or (II) asan emulsifier and additionally comprising a non-ionic surfactant,typically in an amount of 1 to 12% by weight based on the weight offluoropolymer solids.

The aqueous emulsion polymerization of fluorinated monomers, includinggaseous fluorinated monomers, can be conducted using one or moreperfluoropolyethers according to formula (I) and/or (II) as emulsifier.In one particular embodiment, the polymerization may be carried outusing a perfluoropolyether or mixture of perfluoropolyethers accordingto formula (I). In another embodiment, a perfluoropolyether or mixtureof perfluoropolyethers according to formula (II) is used. In yet anotherembodiment, a mixture of one or more perfluoropolyethers according toformula (I) and one or more perfluoropolyethers according to formula(II) is used.

Perfluoropolyethers of formula (I) are commercially available from AnlesLtd., St. Petersburg, Russia. These compounds may be prepared forexample as described by Ershov and Popova in Fluorine Notes 4(11), 2002.Also, these perfluoropolyethers typically form as byproducts in themanufacturing of hexafluoropropylene oxide by direct oxidation ofhexafluoropropylene.

Perfluoropolyethers according to formula (II) can be derived fromreactants that are also used in the manufacturing of fluorinated vinylethers as described in U.S. Pat. No. 6,255,536. Accordingly, theseperfluoropolyethers can be obtained in an economically attractive way asthey can be derived from other starting products that may be used andneeded in the manufacturing of fluoromonomers and fluoropolymers.

In accordance with the present invention, the perfluoropolyether is usedin the aqueous emulsion polymerization of one or more fluorinatedmonomers, in particular gaseous fluorinated monomers. By gaseousfluorinated monomers is meant monomers that are present as a gas underthe polymerization conditions. In a particular embodiment, thepolymerization of the fluorinated monomers is started in the presence ofthe perfluoropolyether, i.e. the polymerization is initiated in thepresence of the perfluoropolyether. The amount of perfluoropolyethersurfactant used may vary depending on desired properties such as amountof solids, particle size etc. Generally the amount of perfluoropolyethersurfactant will be between 0.01% by weight based on the weight of waterin the polymerization and 5% by weight, for example between 0.05% byweight and 2% by weight. A practical range is between 0.05% by weightand 1% by weight. While the polymerization is generally initiated in thepresence of the perfluoropolyether surfactant, it is not excluded to addfurther perfluoropolyether surfactant during the polymerization althoughsuch will generally not be necessary. Nevertheless, it may be desirableto add certain monomer to the polymerization in the form of an aqueousemulsion. For example, fluorinated monomers and in particularperfluorinated co-monomers that are liquid under the polymerizationconditions may be advantageously added in the form of an aqueousemulsion. Such emulsion of such co-monomers is preferably prepared usingthe perfluoropolyether as an emulsifier.

The aqueous emulsion polymerization may be carried out at a temperaturesbetween 10 to 100° C., preferably 30° C. to 80° C. and the pressure istypically between 2 and 30 bar, in particular 5 to 20 bar. The reactiontemperature may be varied during the polymerization to influence themolecular weight distribution, i.e., to obtain a broad molecular weightdistribution or to obtain a bimodal or multimodal molecular weightdistribution.

The aqueous emulsion polymerization is typically initiated by aninitiator including any of the initiators known for initiating a freeradical polymerization of fluorinated monomers. Suitable initiatorsinclude peroxides and azo compounds and redox based initiators. Specificexamples of peroxide initiators include, hydrogen peroxide, sodium orbarium peroxide, diacylperoxides such as diacetylperoxide, disuccinylperoxide, dipropionylperoxide, dibutyrylperoxide, dibenzoylperoxide,benzoylacetylperoxide, diglutaric acid peroxide and dilaurylperoxide,and further per-acids and salts thereof such as e.g. ammonium, sodium orpotassium salts. Examples of per-acids include peracetic acid. Esters ofthe peracid can be used as well and examples thereof includetert.-butylperoxyacetate and tert.-butylperoxypivalate. Examples ofinorganic include for example ammonium-alkali- or earth alkali salts ofpersulfates, permanganic or manganic acid or manganic acids. Apersulfate initiator, e.g. ammonium persulfate (APS), can be used on itsown or may be used in combination with a reducing agent. Suitablereducing agents include bisulfites such as for example ammoniumbisulfite or sodium metabisulfite, thiosulfates such as for exampleammonium, potassium or sodium thiosulfate, hydrazines, azodicarboxylatesand azodicarboxyldiamide (ADA). Further reducing agents that may be usedinclude sodium formaldehyde sulfoxylate (Rongalit®) or fluoroalkylsulfinates as disclosed in U.S. Pat. No. 5,285,002. The reducing agenttypically reduces the half-life time of the persulfate initiator.Additionally, a metal salt catalyst such as for example copper, iron orsilver salts may be added. The amount of initiator may be between 0.01%by weight (based on the fluoropolymer solids to be produced) and 1% byweight. In one embodiment, the amount of initiator is between 0.05 and0.5% by weight. In another embodiment, the amount may be between 0.05and 0.3% by weight.

The aqueous emulsion polymerization system may further comprise othermaterials, such as buffers and, if desired, complex-formers orchain-transfer agents. Examples of chain transfer agents that can beused include dimethyl ether, methyl t-butyl ether, alkanes having 1 to 5carbon atoms such as ethane, propane and n-pentane, halogenatedhydrocarbons such as CCl₄, CHCl₃ and CH₂Cl₂ and hydrofluorocarboncompounds such as CH₂F—CF₃ (R134a).

Examples of fluorinated monomers that may be polymerized using theperfluoropolyether surfactant as an emulsifier include partially orfully fluorinated gaseous monomers including fluorinated olefins such astetrafluoroethylene, chlorotrifluoroethylene, hexafluoropropylene, vinylfluoride, vinylidene fluoride, partially or fully fluorinated allylethers and partially or fully fluorinated vinyl ethers. Thepolymerization may further involve non-fluorinated monomers such asethylene and propylene.

Further examples of fluorinated monomers that may be used in the aqueousemulsion polymerization according to the invention include thosecorresponding to the formula:CF₂═CF—O—R_(f)  (III)wherein R_(f) represents a perfluorinated aliphatic group that maycontain one or more oxygen atoms. Preferably, the perfluorovinyl etherscorrespond to the general formula:CF₂═CFO(R_(f)O)_(n)(R′_(f)O)_(m)R″_(f)  (IV)wherein R_(f) and R′_(f) are different linear or branchedperfluoroalkylene groups of 2-6 carbon atoms, m and n are independently0-10, and R″_(f) is a perfluoroalkyl group of 1-6 carbon atoms. Examplesof perfluorovinyl ethers according to the above formulas includeperfluoro-2-propoxypropylvinyl ether (PPVE-2),perfluoro-3-methoxy-n-propylvinyl ether, perfluoro-2-methoxy-ethylvinylether, perfluoromethylvinyl ether (PMVE), perfluoro-n-propylvinyl ether(PPVE-1) andCF₃—(CF₂)₂—O—CF(CF₃)—CF₂—O—CF(CF₃)—CF₂—O—CF═CF₂.

Still further, the polymerization may involve comonomers that have afunctional group such as for example a group capable of participating ina peroxide cure reaction. Such functional groups include halogens suchas Br or I as well as nitrile groups. Specific examples of suchcomonomers that may be listed here include

(a) bromo- or iodo-(per)fluoroalkyl-(per)fluorovinylethers having theformula:Z—R_(f)—O—CX═CX₂wherein each X may be the same or different and represents H or F, Z isBr or I, R_(f) is a (per)fluoroalkylene C₁-C₁₂, optionally containingchlorine and/or ether oxygen atoms; for example: BrCF₂—O—CF═CF₂,BrCF₂CF₂—O—CF═CF₂, BrCF₂CF₂CF₂—O—CF═CF₂, CF₃CFBrCF₂—O—CF═CF₂, and thelike; and

(b) bromo- or iodo containing fluoroolefins such as those having theformula:Z′—(R_(f)′)_(r)—CX═CX₂,wherein each X independently represents H or F, Z′ is Br or I, R_(f)′ isa perfluoroalkylene C₁-C₁₂, optionally containing chlorine atoms and ris 0 or 1; for instance: bromotrifluoroethylene,4-bromo-perfluorobutene-1, and the like; or bromofluoroolefins such as1-bromo-2,2-difluoroethylene and 4-bromo-3,3,4,4-tetrafluorobutene-1.Examples of nitrile containing monomers that may be used include thosethat correspond to one of the following formulas:CF₂═CF—CF₂—O—R_(f)—CNCF₂═CFO(CF₂)_(L)CNCF₂═CFO [CF₂CF(CF₃)O]_(g)(CF₂)_(v)OCF(CF₃)CNCF₂═CF [OCF₂CF(CF₃)]_(k)O(CF₂)_(u)CNwherein L represents an integer of 2 to 12; g represents an integer of 0to 4; k represents 1 or 2; v represents an integer of 0 to 6; urepresents an integer of 1 to 6, R_(f) is a perfluoroalkylene or abivalent perfluoroether group. Specific examples of nitrile containingliquid fluorinated monomers includeperfluoro(8-cyano-5-methyl-3,6-dioxa-1-octene), CF₂═CFO(CF₂)₅CN, andCF₂═CFO(CF₂)₃OCF(CF₃)CN.

In accordance with a particular embodiment, a fluorinated liquid may beadded to the polymerization system. By the term ‘liquid’ is meant thatthe compound should be liquid at the conditions of temperature andpressure employed in the polymerization process. Typically thefluorinated liquid has a boiling point of at least 50° C., preferably atleast 80° C. at atmospheric pressure. Fluorinated liquids include inparticular highly fluorinated hydrocarbons as well as liquid fluorinatedmonomers. The term ‘highly fluorinated’ in connection with the presentinvention is used to indicate compounds in which most and preferably allhydrogen atoms have been replaced with fluorine atoms as well ascompounds wherein the majority of hydrogen atoms have been replaced withfluorine atoms and where most or all of the remainder of the hydrogenatoms has been replaced with bromine, chlorine or iodine. Typically, ahighly fluorinated compound in connection with this invention will haveonly few, e.g., 1 or 2 hydrogen atoms replaced by a halogen other thanfluorine and/or have only one or two hydrogen atoms remaining. When notall hydrogen atoms are replaced by fluorine or another halogen, i.e.,the compound is not perfluorinated, the hydrogen atoms should generallybe in a position on the compound such that substantially no chaintransfer thereto occurs, i.e., such that the compound acts as an inertin the polymerization, i.e., the compound does not participate in thefree radical polymerization. Compounds in which all hydrogens have beenreplaced by fluorine and/or other halogen atoms are herein referred toas ‘perfluorinated’.

Liquid and fluorinated hydrocarbon compounds that can be used asfluorinated liquid, typically comprise between 3 and 25 carbon atoms,preferably between 5 and 20 carbon atoms and may contain up to 2heteroatoms selected from oxygen, sulfur or nitrogen. Preferably thehighly fluorinated hydrocarbon compound is a perfluorinated hydrocarboncompound. Suitable perfluorinated hydrocarbons include perfluorinatedsaturated linear, branched and/or cyclic aliphatic compounds such as aperfluorinated linear, branched or cyclic alkane; a perfluorinatedaromatic compound such as perfluorinated benzene, or perfluorinatedtetradecahydro phenanthene. It can also be a perfluorinated alkyl aminesuch as a perfluorinated trialkyl amine. It can further be aperfluorinated cyclic aliphatic, such as decalin; and preferably aheterocyclic aliphatic compound containing oxygen or sulfur in the ring,such as perfluoro-2-butyl tetrahydrofuran.

Specific examples of perfluorinated hydrocarbons includeperfluoro-2-butyltetrahydrofuran, perfluorodecalin,perfluoromethyldecalin, perfluoromethylcyclohexane,perfluoro(1,3-dimethylcyclohexane),perfluorodimethyldecahydronaphthalene, perfluorofluorene,perfluoro(tetradecahydrophenanthrene), perfluorotetracosane,perfluorokerosenes, octafluoronaphthalene, oligomers ofpoly(chlorotrifluoroethylene), perfluoro(trialkylamine) such asperfluoro(tripropylamine), perfluoro(tributylamine), orperfluoro(tripentylamine), and octafluorotoluene, hexafluorobenzene, andcommercial fluorinated solvents, such as Fluorinert FC-75, FC-72, FC-84,FC-77, FC-40, FC-43, FC-70, FC 5312 or FZ 348 all produced by 3MCompany. A suitable inert liquid and highly fluorinated hydrocarboncompound isC₃F₇—O—CF(CF₃)—CF₂—O—CHF—CF₃.The fluorinated liquid may also comprise liquid fluorinated monomeralone or in combination with above described liquid fluorinatedcompounds. Examples of liquid fluorinated monomers include monomers thatare liquid under the polymerization conditions and that are selectedfrom (per)fluorinated vinyl ethers, (per)fluorinated allyl ethers and(per)fluorinated alkyl vinyl monomers.

When a fluorinated liquid is used, it will generally be preferred toemulsify the fluorinated liquid. Preferably, the fluorinated liquid isemulsified using the perfluoropolyether surfactant. Also, when afluorinated liquid is used in the polymerization, it will beadvantageous that at least a portion thereof or all is provided at thestart of the polymerization such that the polymerization is initiated inthe presence of the emulsified fluorinated liquid. The use of thefluorinated liquid may improve such properties as the rate ofpolymerization, incorporation of co-monomers and may reduce the particlesize and/or improve the amount of solids that can be obtained at the endof the polymerization.

The aqueous emulsion polymerization may be used to produce a variety offluoropolymers including perfluoropolymers, which have a fullyfluorinated backbone, as well as partially fluorinated fluoropolymers.Also the aqueous emulsion polymerization may result in melt-processiblefluoropolymers as well as those that are not melt-processible such asfor example polytetrafluoroethylene and so-called modifiedpolytetrafluoroethylene. The polymerization process can further yieldfluoropolymers that can be cured to make fluoroelastomers as well asfluorothermoplasts. Fluorothermoplasts are generally fluoropolymers thathave a distinct and well noticeable melting point, typically in therange of 60 to 340° C. or between 100 and 320° C. They thus have asubstantial crystalline phase. Fluoropolymers that are used for makingfluoroelastomers typically are amorphous and/or have a neglectableamount of crystallinity such that no or hardly any melting point isdiscernable for these fluoropolymers.

The aqueous emulsion polymerization results in a dispersion of thefluoropolymer in water. Generally the amount of solids of thefluoropolymer in the dispersion directly resulting from thepolymerization will vary between 3% by weight and about 40% by weightdepending on the polymerization conditions. A typical range is between 5and 30% by weight. The particle size (volume average particle size) ofthe fluoropolymer is typically between 50 nm and 350 nm with a typicalparticle size being between 100 nm and about 300 nm. The amount ofperfluoropolyether according to formula (I) and/or (II) in the resultingdispersion is typically between 0.001 and 5% by weight based on theamount of fluoropolymer solids in the dispersion. A typical amount maybe from 0.01 to 2% by weight or from 0.02 to 1% by weight.

The fluoropolymer may be isolated from the dispersion by coagulation ifa polymer in solid form is desired. Also, depending on the requirementsof the application in which the fluoropolymer is to be used, thefluoropolymer may be post-fluorinated so as to convert any thermallyunstable end groups into stable CF₃ end groups. The fluoropolymer may bepost-fluorinated as described in for example EP 222945. Generally, thefluoropolymer will be post fluorinated such that the amount of endgroups in the fluoropolymer other than CF₃ is less than 80 per millioncarbon atoms.

For coating applications, an aqueous dispersion of the fluoropolymer isdesired and hence the fluoropolymer will not need to be separated orcoagulated from the dispersion. To obtain a fluoropolymer dispersionsuitable for use in coating applications such as for example in theimpregnation of fabrics or in the coating of metal substrates to makefor example cookware, it will generally be desired to add furtherstabilizing surfactants and/or to further increase the fluoropolymersolids. For example, non-ionic stabilizing surfactants may be added tothe fluoropolymer dispersion. Typically these will be added thereto inan amount of 1 to 12% by weight based on fluoropolymer solids. Examplesof non-ionic surfactants that may be added includeR¹—O—[CH₂CH₂O]_(n)—[R²O]_(m)—R³  (V)wherein R¹ represents an aromatic or aliphatic hydrocarbon group havingat least 8 carbon atoms, R² represents an alkylene having 3 carbonatoms, R³ represents hydrogen or a C₁-C₃ alkyl group, n has a value of 0to 40, m has a value of 0 to 40 and the sum of n+m being at least 2. Itwill be understood that in the above formula (V), the units indexed by nand m may appear as blocks or they may be present in an alternating orrandom configuration. Examples of non-ionic surfactants according toformula (V) above include alkylphenol oxy ethylates such as ethoxylatedp-isooctylphenol commercially available under the brand name TRITON™such as for example TRITON™ X 100 wherein the number of ethoxy units isabout 10 or TRITON™ X 114 wherein the number of ethoxy units is about 7to 8. Still further examples include those in which R¹n the aboveformula (V) represents an alkyl group of 4 to 20 carbon atoms, m is 0and R³ is hydrogen. An example thereof includes isotridecanolethoxylated with about 8 ethoxy groups and which is commerciallyavailable as GENAPOL®X080 from Clariant GmbH. Non-ionic surfactantsaccording to formula (V) in which the hydrophilic part comprises ablock-copolymer of ethoxy groups and propoxy groups may be used as well.Such non-ionic surfactants are commercially available from Clariant GmbHunder the trade designation GENAPOL® PF 40 and GENAPOL® PF 80.

The amount of fluoropolymer solids in the dispersion may beupconcentrated as needed or desired to an amount between 30 and 70% byweight. Any of the known upconcentration techniques may be usedincluding ultrafiltration and thermal upconcentration.

The obtained fluoropolymer may be conveniently used in most applicationsoptionally after the addition of non-ionic surfactant and/orupconcentration and without removing the perfluoropolyether surfactant.Nevertheless, for reasons of for example costs, it may be desirable toremove the perfluoropolyether from the dispersion. It has been foundthat the perfluoropolyether surfactant can be readily removed from theaqueous dispersion using an anion exchange resin. Accordingly, anon-ionic surfactant, e.g. as disclosed above is added to thefluoropolymer dispersion, generally in an amount of 1 to 12% by weightand the fluoropolymer dispersion is then contacted with an anionexchange resin. Such a method is disclosed in detail in WO 00/35971. Theanion exchange process is preferably carried out in essentially basicconditions. Accordingly, the ion exchange resin will preferably be inthe OH— form although anions like fluoride or sulfate may be used aswell. The specific basicity of the ion exchange resin is not verycritical. Strongly basic resins are preferred because of their higherefficiency. The process may be carried out by feeding the fluoropolymerdispersion through a column that contains the ion exchange resin oralternatively, the fluoropolymer dispersion may be stirred with the ionexchange resin and the fluoropolymer dispersion may thereafter beisolated by filtration. The perfluoropolyether surfactant maysubsequently be recovered from the anion exchange resin by eluting theloaded resin. A suitable mixture for eluting the anion exchange resin isa mixture of ammonium chloride, methanol and water.

EXAMPLES

Test Methods:

The latex particle size determination was conducted by means of dynamiclight scattering with a Malvern Zetazizer 1000 HSA in accordance toISO/DIS 13321. Prior to the measurements, the polymer latexes as yieldedfrom the polymerisations were diluted with 0.001 mol/L KCl-solution, themeasurement temperature was 25° C. in all cases. The reported average isthe Z-average particle diameter.

SSG

Polymer Density was measured according to ASTM4894 Method D792.

The Polymerization

The polymerization experiments were performed in a 40 L autoclaveequipped with an impeller agitator and a baffle. The autoclave wasevacuated and than charged with 33 l of deionized water and set to 35°C. Agitation was started at 160 rpm and in three following cycles, thevessel was evacuated and subsequently charged with nitrogen to assurethat all oxygen had been removed. Another cleaning cycle pas performedusing TFE. After pressurizing to 0.2 MPA the TFE was released tocombustion and the reactor was evacuated again. Then 140 mmolfluorinated emulsifier as specified in table 1 and the followingmaterials were added: 24 mg of Cupper sulfate penta hydrate, 0.6 mg ofsulphuric acid and 8 g of a 25% by weight of aqueous ammonia solutionand 5.6 g of PPVE-2 mixed with a small amount of water. Finally thereactor was pressurized with TFE to 0.2 MPA and 50 g of HFP were added.The reactor was than set to 1.5 MPa using TFE and 200 ml of an aqueousinitiator solution containing 187 mg of sodium sulfite and 429 mg ofammonium peroxodisulfate was pumped into the vessel The beginning of thepolymerization is indicated by a pressure drop. During polymerizationthe pressure was maintained at 1.5 MPa by feeding TFE into the gasphase. After 3.64 kg of TFE had been added, the monomer valve wasclosed. The characteristics of the obtained polymer dispersion aresummarized in table 1.

1000 ml of this polymer dispersion was coagulated by adding 20 mlhydrochloric acid under agitation. When coagulation was performed 100 mlof benzene were added and stirred again. After dewatering, the latex waswashed several times with deionized water. The polymer was driedovernight at 100° C. in a vacuum oven. Comparative Example Example 1Formula C₇F₁₅COONH₄ CF₃OC₃F₆OCF(CF₃)COONH₄ Polymerization 81 66 Time(min.) Average Particle 103 109 Size (nm) SSG 2.148 2.154 g/cm³ Solidscontent (% 9.9 9.8 by weight)

1. Method for making a fluoropolymer comprising an aqueous emulsionpolymerization of one or more fluorinated monomers wherein said aqueousemulsion polymerization is carried out in the presence of aperfluoropolyether as an emulsifier, said perfluoropolyether beingselected from the group consisting of perfluoropolyethers according toformula (I):CF₃—(OCF₂)_(m)—O—CF₂—X  (I) wherein m has a value of 1 to 6 and Xrepresents a carboxylic acid group or salt thereof, perfluoropolyethersaccording to formula (II):CF₃—O—(CF₂)₃—(OCF(CF₃)—CF₂)_(z)—O-L-Y  (II) wherein z has a value of 0,1, 2 or 3, L represents a divalent linking group selected from—CF(CF₃)—, —CF₂— and —CF₂CF₂— and Y represents a carboxylic acid groupor salt thereof, and mixtures of perfluoropolyethers according toformula (I), (II), or combination thereof.
 2. Method according to claim1 wherein said one or more fluorinated monomers comprise one or moregaseous fluorinated monomers.
 3. Method according to claim 1 whereinsaid one or more fluorinated monomers comprise perfluorinated monomers.4. Method according to claim 1 wherein said aqueous emulsionpolymerization is carried out in the presence of a fluorinated liquidand wherein said fluorinated liquid is emulsified using saidperfluoropolyether as an emulsifier.
 5. Method according to claim 1wherein said aqueous emulsion polymerization is carried out using saidperfluoropolyether as the only emulsifier.
 6. Method according to claim1 wherein the amount of said perfluoropolyether is between 0.01 and 5%by weight based on the amount of water in the emulsion polymerization.7. A dispersion comprising an aqueous dispersion of a fluoropolymer anda perfluoropolyether as an emulsifier, said perfluoropolyether beingselected from the group consisting of perfluoropolyethers according toformula (I):CF₃—(OCF₂)_(m)—O—CF₂—X  (I) wherein m has a value of 1 to 6 and Xrepresents a carboxylic acid group or salt thereof, perfluoropolyethersaccording to formula (II):CF₃—O—(CF₂)₃—(OCF(CF₃)—CF₂)_(z)—O-L-Y  (II) wherein z has a value of 0,1, 2 or 3, L represents a divalent linking group selected from—CF(CF₃)—, —CF₂— and —CF₂CF₂— and Y represents a carboxylic acid groupor salt thereof, and mixtures of perfluoropolyethers according toformula (I), (II), or combination thereof.
 8. A dispersion according toclaim 7 wherein said dispersion is free of perfluoroalkanoic acids orsalts thereof.
 9. A dispersion according to claim 7 wherein the amountof said perfluoropolyether is between 0.001 and 5% by weight based onthe fluoropolymer solids.
 10. A dispersion according to claim 7 whereinthe amount of fluoropolymer solids is between 10 and 30% by weight. 11.A dispersion according to claim 7 wherein the amount of solids is morethan 30 and up to 70% by weight.
 12. A dispersion according to claim 7wherein the dispersion further comprises a non-ionic surfactant.
 13. Amethod comprising coating or impregnating a substrate using an aqueousdispersion as defined in claim 7.